The present invention relates to a method and apparatus for recovering metal values from particulate metal-containing dust, and more particularly to a method and apparatus for recovering metal from electric arc furnace dust.
Rotary Hearth Furnace processes such as the Midrex FASTMET(trademark) Process, INMETCO process, and Maumee process, and Rotary Kiln processes such as Horse Head and BUS Waelz Kiln, have been developed, marketed, and sold as methods of recovering iron units, zinc and other valuable metallics from large integrated steel mill waste streams. Economical plant capacity for these processes is in the range of 100,000 to 200,000 tonnes of waste processed per year. Mini mills, based on Electric Arc Furnace technology, are typically smaller in capacity than integrated steels mills and produce significantly less waste, ranging from 5,000 to 30,000 tonnes per year. The specific dust waste from Electric Arc Furnace (EAF) operations has been classified by the US Environmental Protection Agency (US EPA) as K061, a hazardous waste requiring special handling, inventory control and approved disposal by US EPA. These special handling requirements and implied liabilities make operation of a large capacity centralized EAF Dust Processing facility cumbersome and undesirable. High Temperature Metals Recovery (HTMR) processing of wastes classified as K061 has been identified by US EPA as the preferred method of treating and delisting.
It is therefore desirable to provide an economical method of thermal treatment of as-generated mini-mill waste located at the site at which such waste is produced.
In 1998, Midrex International BV received U.S. Pat. No. 5,730,775, that teaches an improved method known by the trade name or trademark of FASTMET(trademark), and apparatus for producing direct reduced iron from dry iron oxide and carbon compacts that are layered no more than two layers deep onto a rotary hearth, and are metallized by heating the compacts to temperatures of approximately 1316xc2x0 to 1427xc2x0 C., for a short time period. For a general understanding of the recent art, U.S. Pat. No. 5,730,775 is herein incorporated by reference.
In existing rotary hearth and rotary kiln HTMR processes, most of the capital cost is associated with feed preparation equipment necessary to feed the processing furnace, with the fabrication and erection of the furnace itself, and with product handling equipment.
The invention provides a method and apparatus for recovering principally iron and zinc values from EAF dust. The method of recovering metal values from metal-containing dust, comprises the steps of:
a. mixing metal-containing dust and carbonaceous fines to form a particulate mixture;
b. heating the metal-containing dust and carbonaceous fines mixture on a moving bed horizontal tunnel furnace at a sufficient temperature and for a sufficient time to reduce and release volatile metals and alkali metals therefrom along with gaseous products;
c. collecting the released metals and gases, and reoxidizing the metals; and
d. separating the metal values from the gases and removing the metal values, principally zinc and iron, from the process as product.
The furnace is sealed to prevent the intrusion of ambient air. A baghouse is provided to collect the volatile metals.
The invented process feeds a simple xe2x80x9cas isxe2x80x9d mixture of EAF dust and carbon fines (coal dust, charcoal, pet coke. etc.) through a feed leg onto a horizontal stroke conveyor. Motion of the conveyor controls the feed rate and distributes the feed evenly across the conveyor pan. Since the pan is not a xe2x80x9cmoving hearthxe2x80x9d the feed area is always cold,. i.e., at ambient temperature. This eliminates the complicated and expensive feed and leveling systems associated with rotary hearth processes.
The principal object of the present invention is to provide an improved method of achieving rapid and efficient reduction of metal oxide fines and recovery of metal values therefrom.
Another object of the invention is to provide a simple, low-cost method and apparatus for the processing of EAF dust.
Another object of the invention is to provide a reduction furnace apparatus which can be installed on site, or can be fully portable.
A further object of the invention is to provide a fines reduction furnace capable of operating at variable speed
Another object of the invention is to provide means for recovering useable zinc oxide from EAF dust.
Another object of the invention is to provide a method which requires no hazardous waste water treatment.
Another object of the invention is to provide a method which produces no hazardous solid waste.